1. Field of the Invention
The present invention relates to a laser diode driving circuit.
2. Description of the Prior Art
FIG. 1 shows a conventional laser diode driving circuit. Numeral 1 is a constant current source whose one end is connected to a direct current power source line 2. The other end thereof is connected through a differential circuit 3 to a ground line 4. The differential circuit 3 includes a first series circuit 5 and a second series circuit 6. The first series circuit 5 includes a first switching transistor Q1 whose base is provided with a constant voltage E and a laser diode LD connected to the emitter of the transistor Q1.
The second series circuit 6 includes a second switching transistor Q2 whose base is provided with a driving pulse P and a resistor R connected to the emitter of the transistor Q2. The pulse width of the driving pulse P is extremely small. While the pulse P is being outputted, the first switching transistor Q1 is OFF since the second switching transistor Q2 is ON. As a result, the laser diode LD is not ON. While the pulse P is not being outputted, since the second switching transistor Q2 is OFF and the first switching transistor Q1 is ON, the laser diode LD is ON. As a result, laser beams are irradiated.
When the speed of response of the constant current source 1 is lower than that of the differential circuit 3, if a voltage Vop obtained between the anode and cathode of the laser diode when the laser diode LD is ON differs from a voltage V across the resistor R, it is impossible to desirably drive the laser diode LD by means of a pulse. That is, when V&lt;Vop, a current I.sub.2 which flows when the transistor Q2 is ON is larger than a current I.sub.1 which flows when the transistor Q1 is ON; however, when the transistor Q2 is turned off and the transistor Q1 is turned on, the output current from the constant current source 1 does not immediately become the current I.sub.1. At first, the current I.sub.2 flows, and then, the output current gradually decreases to the current I.sub.1.
Conversely, when V&gt;Vop, I.sub.2 &lt;I.sub.1. In this case, when the transistor Q2 is turned off and the transistor Q1 is turned on, the output current of the constant current source 1 gradually increases from I.sub.2 to I.sub.1. FIG. 2 graphically shows the above increase and decrease of I.sub.2. I.sub.2 shows the former case, while I.sub.2 ' shows the latter case.
As described above, the currents which flow through the laser diode LD are not constant; they transiently decrease or increase. Since it is impossible to allow a constant current to flow while the laser diode is ON, it is impossible to drive the laser diode by means of an ideal pulse current waveform.